Dual-fuel engines are well known in the art and combust a mixture of two different types of fuel. One exemplary dual-fuel engine combusts a mixture of liquid fuel (e.g., a diesel fuel) and gaseous fuel (e.g., natural gas). By combusting two different types of fuel, advantages of both fuels (e.g., efficiency, power, emissions, cost, etc.) can be realized. For example, diesel fuel may be more power-dense and, thus, generate a greater amount of power per volume of fuel consumed. Natural gas, however, may be more abundant and therefore less expensive than diesel fuel. In addition, natural gas may burn cleaner in some applications.
In order to realize full benefits of operating a dual-fuel engine, care should be taken to ensure proper combustion of the different fuels. That is, diesel fuel may be compression-ignited at compression ratios of about 18:1, while natural gas may ignite at compression ratios that are much lower (e.g., at about 12:1). Accordingly, when natural gas is introduced into a diesel engine having high compression ratios, pre-ignition (a.k.a., knocking) of the natural gas can occur. This pre ignition can reduce an engine's efficiency, increase noise, and/or cause damage to the engine.
An exemplary dual-fuel engine is disclosed in JP Patent 2008/202545 (“the '545 patent”). The engine includes an intake valve driving means that drives an intake valve to open and close a port of a combustion chamber. A closing of the intake valve is selectively accelerated by the intake valve driving means during operation of a premixed combustion mode, when compared to a diffusion combustion mode. This accelerated closing adjusts the compression ratio of the engine to improve heat efficiency or fuel ignitability, thereby preventing the occurrence of knocking during premixed combustion.
Although the intake valve driving means of the '545 patent may be capable of adjusting the compression ratio of a dual fuel engine, it may lack broad applicability. Specifically, because the compression ratio adjustment is achieved via accelerated closing of an intake valve, the intake valve driving means may not be useful in an engine that does not have intake valves. That is, the intake valve driving means may lack applicability in a two stroke engine. Further, by adjusting intake valve operation, it may be possible to cause an undesired increase in pressure, temperature, and/or fuel concentration within an associated intake air box or manifold that could make engine operation unstable. In addition, accelerating only the closing of a valve may not provide enough flexibility to control the compression ratio in all situations.
The engine of the present disclosure solves one or more of the problems set forth above and/or other problems in the art.